JP2010199214A - Mems tunable capacitor - Google Patents

Abstract

There is provided a MEMS tunable capacitor in which a distance between a movable electrode and a fixed electrode is not easily changed by application of a voltage.
A device 10 includes a signal line 12 formed on a surface P of a substrate, two or more columnar anchors 20 standing between the signal line 12 and a position sandwiching the signal line 12. The plate-shaped fixed electrode 14 is provided, and a plate-shaped movable electrode 16 suspended by a spring 18 extending from the anchor 20 so as to be sandwiched by the fixed electrode 14 in the space on the signal line 12. The anchor 20 is fixed at an outer position of the fixed electrode 14 in the vicinity of both ends in the longitudinal direction of the signal line 12, and a spring 18 is provided at the tip so as to straddle the signal line 12. A pair of springs 18 extending from the pair of anchors 20 across the signal line 12 suspends the movable electrode 16 provided along the longitudinal direction of the signal line 12 in the space above the signal line 12 and is movable. The electrode 16 is located in the middle of the fixed electrode 14 and in parallel therewith.
[Selection] Figure 3

Description

  The present invention relates to a MEMS tunable capacitor.

  Since the next-generation mobile phone uses a large number of frequency bands, a receiver that can receive a plurality of frequencies is required. As a simple method, a method of mounting and using a plurality of components (for example, capacitors, inductors, etc.) in the RF front end can be considered. However, in this case, the volume of the RF front end is increased and the cost is increased. .

  On the other hand, a method using a MEMS tunable capacitor capable of having a plurality of impedances in one device has attracted attention. In particular, when a tunable matching circuit is combined with an antenna or an amplifier like the mobile phone shown as an example in FIG.

  There has been proposed a MEMS variable capacitance element that includes a movable electrode and a fixed electrode and makes the capacitance variable by moving the movable electrode with an electrostatic force (for example, see Patent Documents 1 to 3).

  The content disclosed in Patent Document 1 is a configuration in which the movable electrode is contacted and separated between a pair of fixed electrodes arranged side by side. The content disclosed in Patent Document 2 includes a movable beam and a fixed electrode provided on separate substrates. The contents disclosed in Patent Document 3 include a fixed electrode fixed to the main surface of the substrate and a movable electrode movable relative to the fixed electrode. In the configuration in which the capacitance is varied by changing the distance between the fixed electrode and the movable electrode as described above, it is essential to form an insulating film on the surfaces of both electrodes in order to prevent a short circuit between the electrodes.

  However, in the variable capacitance element as described above, stiction due to charge trapping of the insulating film becomes a problem. That is, if the voltage is continuously applied for a long time to fix the capacitance value of the variable capacitance element, the charge in the insulating film between the electrodes is trapped. No (fixed) state. This is a problem caused by changing the capacitance by applying a voltage and changing the distance between the movable electrode and the fixed electrode by electrostatic force. It is difficult to achieve a proper solution.

  In addition, a configuration employing an IBA driving method in order to prevent stiction as described above is disclosed (for example, see Non-Patent Document 1). When the IBA drive method detects the amount of charge trapped in the insulating layer and determines that the applied voltage does not leave the actuator immediately (stiction state), the trap charge of the insulating layer is applied by applying a reverse voltage. To prevent stiction.

  However, the following demerits can be considered by adopting the above-mentioned IBA driving method for the MEMS variable capacitance element. That is, since the IBA drive circuit for preventing stiction occupies a large area, there arises a problem that the area of the entire chip increases. In addition, there is a problem that the structure becomes complicated and the cost for an additional device such as an IBA drive circuit is required.

JP 2004-1209 A JP-T-2006-518926 JP 2008-221398 A "RF MEMS Variable Capacitance Element Toshiba Review Vol.63 No.2", Toshiba Corporation, 2008, p32-p36

  The above problem is caused by a structure in which the fixed electrode and the movable electrode are too close to each other or may come into contact with each other by applying a voltage. Keep the distance constant.

  An object of the present invention is to provide a MEMS tunable capacitor in which the distance between the movable electrode and the fixed electrode is unlikely to change due to application of a voltage.

  The MEMS tunable capacitor according to claim 1, a signal line provided on a substrate, a pair of fixed electrodes provided on the substrate and electrically connected across the signal line, and having a rectangular parallelepiped shape, An electrostatic force generated by providing a potential difference between the fixed electrode and the movable electrode, the movable electrode being suspended between the fixed electrode facing the signal line and being movable toward and away from the signal line. The electric capacity is changed by changing a distance between the variable electrode and the signal line.

  According to the above invention, compared to the case where this configuration is not adopted, the variable electrode is provided between the pair of fixed electrodes provided on the substrate with the signal line interposed therebetween, and the signal line and the variable electrode are applied by voltage application. By changing the distance between the fixed electrode and the variable electrode, the distance between the fixed electrode and the variable electrode does not fluctuate due to voltage application, so that a variable-capacity MEMS tunable capacitor that hardly causes stiction can be obtained.

  The MEMS tunable capacitor according to claim 2 is the configuration according to claim 1, wherein the end of the movable electrode on the side far from the substrate protrudes beyond the end of the fixed electrode on the side far from the substrate. It is characterized by.

  According to the above invention, compared to the case where this configuration is not adopted, the end of the movable electrode farther from the substrate protrudes than the fixed electrode, so that the electrostatic force received at this portion makes it more movable. The electrode can be displaced.

  The MEMS tunable capacitor according to a third aspect is the structure according to the first or second aspect, wherein the movable electrode is arranged in a direction along the surface of the substrate rather than in a direction along the surface of the fixed electrode. It is characterized by being suspended by an elastic member having high rigidity.

  According to said invention, compared with the case where this structure is not employ | adopted, it can be set as the structure which a movable electrode does not move easily in the direction along the surface of a board | substrate, but can move easily in the direction which contacts / separates from a board | substrate.

  A MEMS tunable capacitor according to a fourth aspect is characterized in that, in the configuration according to the third aspect, the elastic member has a width in a direction along the substrate larger than a thickness in a direction perpendicular to the substrate.

  According to said invention, compared with the case where this structure is not employ | adopted, it can be set as the structure which a movable electrode does not move easily in the direction along the surface of a board | substrate, but can move easily in the direction which contacts / separates from a board | substrate.

  The MEMS tunable capacitor according to claim 5 is the structure according to any one of claims 1 to 4, wherein the surface of the signal line and the surfaces of the fixed electrode and the movable electrode facing each other are insulated. It is characterized by being covered with a film.

  According to said invention, compared with the case where this structure is not employ | adopted, it can be set as the structure which is hard to raise | generate a short circuit even if a movable electrode contacts a signal line and a fixed electrode.

  The MEMS tunable capacitor according to claim 6, a signal line provided on a substrate, a plurality of fixed electrodes provided in a row on the substrate across the signal line and electrically connected, A space on the substrate facing the signal line, a variable capacitance electrode suspended so as to be able to contact with and separate from the signal line, a movable electrode provided integrally with the variable capacitance electrode and positioned between the fixed electrodes, And the capacitance is changed by changing the distance between the variable capacitance electrode and the signal line by an electrostatic force generated by providing a potential difference between the fixed electrode and the variable capacitance electrode.

  According to the above invention, as compared with the case where this configuration is not adopted, the variable capacitance electrode is movable between the plurality of fixed electrodes provided on the substrate across the signal line and movable in the contact / separation direction with the signal line. By providing a voltage between the movable electrode provided integrally with the variable capacitance electrode and the fixed electrode to change the distance between the signal line and the variable capacitance electrode, the fixed electrode Since the distance from the variable electrode does not fluctuate due to voltage application, a configuration in which stiction is difficult to occur can be achieved.

  The MEMS tunable capacitor according to claim 7, wherein the variable capacitance electrode has a rectangular parallelepiped shape provided along the substrate, and a plurality of the fixed electrodes are erected from the substrate on both sides of the signal line. A plurality of thin plates provided on the variable capacitance electrode at equal intervals with the fixed electrode. The comb-like members are arranged in a staggered manner between the fixed electrodes one by one, and are arranged so that the surfaces face each other with a space between each other, and the size of the movable electrode in the direction orthogonal to the substrate is And the end of the movable electrode and the variable capacitance electrode on the side farther from the substrate is the same as the thickness of the variable capacitance electrode on the side farther from the substrate. Characterized in that apart from said substrate than part.

  According to the above invention, as compared with the case where this configuration is not adopted, the comb-shaped movable electrode provided on the variable capacitance electrode movable in the contact and separation direction with the signal line, and the same standing on the substrate The fixed electrodes of the comb-tooth structure are meshed so as not to contact each other, and the overlapping area of the movable electrode and the fixed electrode changes when the variable capacitance electrode moves in the contact / separation direction from the signal line. Since the distance from the movable electrode does not fluctuate due to voltage application, a MEMS tunable capacitor that hardly causes stiction can be obtained.

  The MEMS tunable capacitor according to claim 8 is the configuration according to claim 6 or 7, wherein the variable capacitance electrode has a direction along the surface of the substrate rather than a rigidity along the surface of the fixed electrode. It is characterized by being suspended by an elastic member having high rigidity.

  According to the invention described above, the variable capacitance electrode can be easily moved in the contact / separation direction from the signal line and hardly moved in the direction orthogonal to the signal line as compared with the case where the present configuration is not adopted.

  The MEMS tunable capacitor according to a ninth aspect is characterized in that, in the configuration according to the eighth aspect, the elastic member has a width in a direction along the substrate larger than a thickness in a direction perpendicular thereto.

  According to the invention described above, the variable capacitance electrode can be easily moved in the contact / separation direction from the signal line and hardly moved in the direction orthogonal to the signal line as compared with the case where the present configuration is not adopted.

  The MEMS tunable capacitor according to a tenth aspect is the configuration according to the eighth aspect, wherein the variable capacitance electrode includes a plurality of the elastic members provided in a direction along the signal line and a direction crossing the elastic members. It is supported by a plurality of the elastic members provided on the surface.

  According to the above invention, as compared with the case where this configuration is not adopted, the variable capacitance electrode is elastically extended in the direction along the signal line and in the direction intersecting the signal line in the plane along the substrate. Since it is supported by the member, it can be configured to be difficult to move in a plane along the substrate.

  The MEMS tunable capacitor according to claim 11 is the configuration according to any one of claims 6 to 10, wherein the surface of the signal line, the surface of the variable capacitance electrode, the fixed electrode, and the The surfaces of the movable electrodes facing each other are covered with an insulating film.

  According to the above-described invention, compared to a case where this configuration is not adopted, a configuration in which a short circuit is not easily caused even when the variable capacitance electrode contacts the signal line or the movable electrode contacts the fixed electrode can be achieved.

  A MEMS tunable capacitor according to a twelfth aspect is characterized in that, in the configuration according to any one of the first to eleventh aspects, a voltage is applied to the fixed electrode and the movable electrode is grounded. And

  According to the above invention, if a voltage is applied to the fixed electrode that is not in contact with the movable electrode and the movable electrode side is grounded, even if the movable electrode contacts the signal line through the insulating layer, the movable electrode side Since it is not connected to a power supply, it can be configured to suppress the generation of signal noise due to power supply voltage fluctuations.

  As described above, the present invention provides a MEMS tunable capacitor in which the distance between the movable electrode and the fixed electrode is unlikely to change due to application of a voltage.

<First Embodiment>
An example of the MEMS tunable capacitor according to the first embodiment of the present invention is shown in FIGS. It should be noted that the numerical values such as the dimensions shown are examples, and the present invention is not limited to these in practice, and can be implemented in various modes without departing from the scope of the invention.

  As shown in FIGS. 2 and 3, the device 10 that is the MEMS tunable capacitor according to the first embodiment of the present invention includes a signal line 12 formed on the surface (mounting surface) P of the substrate, and a signal line 12. A pair of or more columnar anchors 20 standing at a position sandwiching the signal line, a plate-like fixed electrode 14 standing at a position sandwiching the signal line 12, and a spring 18 extending from the anchor 20 12 and a plate-like movable electrode 16 suspended so as to be sandwiched between fixed electrodes 14.

  The anchor 20 is fixed at an outer position of the fixed electrode 14 in the vicinity of both ends in the longitudinal direction of the signal line 12, and a spring 18 is provided at the tip portion so as to straddle the signal line 12. A pair of springs 18 extending from the pair of anchors 20 across the signal line 12 suspends the movable electrode 16 provided along the longitudinal direction of the signal line 12 in the space above the signal line 12. The movable electrode 16 is located in the middle of the fixed electrode 14 and in parallel therewith.

  As shown in FIG. 3, the tip of the movable electrode 16 in the Z direction (end farther from the surface P) than the height of the fixed electrode 14 provided on the surface P in the protruding direction (Z direction) from the surface P is Z. As shown in FIGS. 3A and 3C, the movable electrode 16 is held between the fixed electrodes 14 at a position far from the surface P.

  Since the spring 18 for suspending the movable electrode 16 has a large size in the direction along the surface P (xy direction) and the thickness in the Z direction is thinner than the xy direction, the movable electrode 16 suspended by the spring 18 is in the xy direction. It is difficult to move, and it is held easily in the Z direction that is in contact with and away from the surface P. That is, the movable electrode 16 does not contact or separate in the direction along the surface P (xy direction) with respect to the fixed electrode 14 and is movable in the contact or separation direction (arrow L direction in FIG. 2) with respect to the signal line 12. Has been.

  An insulating layer 30 and an insulating layer 32 are provided on the outer peripheral surface of the signal line 12 and the surface of the fixed electrode 14 near the signal line 12, that is, the surface facing the movable electrode 16, respectively. 12 and the movable electrode 16, and even if the fixed electrode 14 and the movable electrode 16 contact, it is set as the structure which is not short-circuited.

<Example of Manufacturing Process of First Embodiment>
4 and 5 show an example of a process for forming a device which is an example of a MEMS tunable capacitor according to the present invention. The materials and numerical values shown are examples, and the present invention is not limited to these at the time of implementation. The present invention can be implemented in various modes without departing from the gist.

  First, as shown in FIG. 4A, a 100 nm WSi layer 102 for forming a signal line 12 is formed on a silicon substrate 100 by a known method (for example, film formation by sputtering). 4 and 5 show the B-B ′ cross section shown in FIG. 3B and the A-A ′ cross section shown in FIG. 3A side by side.

  Next, as shown in FIG. 4B, the WSi layer 102 is etched to leave the WSi layer 102A that becomes the signal line 12.

  Further, as shown in FIG. 4C, POLy-Si 104 is formed to a thickness of 1 μm by a known method (for example, film formation by LP-CVD).

  Next, as shown in FIG. 4D, POLy-Si 104 is etched, and the remaining portion is formed as a fixed electrode 14.

  Further, as shown in FIG. 4E, a SiN insulating layer 106 is deposited to a thickness of 0.1 μm by a known method (for example, film formation by LP-CVD).

  Next, as shown in FIG. 4F, the SiN insulating layer 106 is etched, the insulating layer 30 is formed on the outer peripheral surface of the signal line 12, and the insulating layer 32 is formed on the surface of the fixed electrode 14 facing the signal line 12. .

Further, as shown in FIG. 4G, a SiO ₂ layer 108 as a sacrificial layer is formed to a thickness of 1, 7 μm by a known method (for example, film formation by PE-CVD).

Next, as shown in FIG. 5A, the primary etching of the SiO ₂ layer 108 is performed with a thickness of 1 μm. Thereby, the space 120 used as the anchor 20 is created.

Further, as shown in FIG. 5B, the secondary etching of the SiO ₂ layer 108 is performed with a thickness of 1.2 μm. As a result, a space 160 to be the movable electrode 16 is created.

Next, as shown in FIG. 5C, the third etching of the SiO ₂ layer 108 is performed with a thickness of 0.2 μm. This creates a space 180 that becomes the spring 18.

  Further, as shown in FIG. 5D, POLy-Si 110 for forming the movable electrode 16 and the anchor 20 is formed to a thickness of 1 μm by a known method (for example, film formation by LP / CVD).

  Next, as shown in FIG. 5E, POLy-Si 110 is polished and planarized by a CMP (Chemical Mechanical Polishing) process or the like.

  Finally, as shown in FIG. 5F, POLy-Si 110 is removed. For example, POLy-Si110 is removed by immersing in 5% HF. By these steps, as shown in FIG. 5F, the movable electrode 16 suspended by the spring 18 is positioned on the signal line 12 covered with the insulating layer 30, and the fixed electrode 14 stands on both sides thereof. It can be set as the structure to install.

<Operation and Effect of First Embodiment>
The operation of the MEMS tunable capacitor according to the first embodiment of the present invention will be described with reference to FIG.

  In order for the movable electrode 16 of the device 10 shown in FIG. 3 to be movable with respect to the signal line 12 without changing the distance from the fixed electrode 14, the movable electrode 16 is in the z direction in FIG. 3, that is, from the surface P of the substrate. It is necessary to have a shape that is easy to move in the contact and separation directions and difficult to move in the x direction and the y direction, which are directions along the surface P of the substrate.

In the following (Equation 1), k is the spring coefficient of the spring 18 (cantilever), E is the Young's modulus of the material of the spring 18 (about 160 GPa for POLYSi), b is the width of the spring 18, and t is the spring 18 Thickness 1 is the length of the spring 18. Kp in (Expression 2) represents the spring coefficient of the entire spring 18 connected in parallel to the movable electrode 16. Kt (of Equation 3) indicates the overall spring coefficient when the spring 18 is connected in series.

  In the example shown in FIG. 3, the length of one spring 18 is 40 μm, the thickness is 0.2 μm, and the width is 5 μm. When calculated by the equation (1), the spring coefficient kz in the z direction is about 0.025 N / m. Since four springs 18 are connected to the movable electrode 16, kpz is about 0.1 N / m.

  With respect to the spring coefficient kz in the z direction, the spring coefficient kx in the x direction can be calculated when the thickness b of the spring 18 is 0.2 μm, the length 1 is 40 μm, and t is 5 μm. Therefore, kx = 15.6 N / m and kpx is about 62.4 N / m. That is, the x-direction spring coefficient kx is approximately 624 times the z-direction spring coefficient kz.

  The spring coefficient ky in the y direction can be calculated when the thickness b of the spring 18 is 0.2 μm, the length 1 is 5 μm, and t is 30 μm. Therefore, the spring coefficient ky is about 4 MN / m and kpy is about 16 MN / m. This is about 160 million times the z-direction spring coefficient kz.

  For this reason, the movable electrode 16 has a shape that is easy to move in the z direction, that is, a direction that is close to and away from the surface P of the substrate, and that is difficult to move in the x and y directions that are along the surface P of the substrate. I can say that.

  As described above, in order to make the movable electrode 16 easy to move in the z direction, the width of the shape of the spring 18 in the direction along the surface P of the substrate (x direction, y direction) is the size in the z direction perpendicular thereto. It is necessary to be sufficiently larger than (thickness).

  In the above example, the ratio of the width in the direction (x direction, y direction) along the surface P of the substrate of the spring 18 to the size (thickness) in the z direction perpendicular thereto is 25, and the spring coefficient kx in the x direction. Is approximately 624 times the spring coefficient kz in the z direction.

  When this ratio is 2, the spring coefficient kx in the x direction is about four times the spring coefficient kz in the z direction. The inventors have obtained knowledge from experiments that it is possible to obtain a selection ratio in the x direction and the z direction even at this magnification. For this reason, in order to make the movable electrode 16 easily move in the z-direction and difficult to move in the x-direction and the y-direction, the width of the spring 18 in the direction along the substrate surface P (x-direction, y-direction) It is desirable that the ratio of the size (thickness) in the z direction perpendicular to is at least 2 or more.

  Next, the operation of the MEMS tunable capacitor according to the first embodiment of the present invention is shown in FIGS.

  6 shows the operating principle of the device 10, FIG. 7 shows the relationship between the displacement of the movable electrode 16 relative to the signal line 12 and the applied voltage, and FIG. 8 shows the relationship between the movable electrode 16 and the signal line 12. The relationship between capacitance change and voltage is shown.

  That is, FIG. 7 shows the displacement in the z direction of the movable electrode 16 when a voltage of 0 V to 3 V is applied to the fixed electrode 14 in the example of the dimensions shown in FIG. Based on FIG. 7, the capacitance change between the movable electrode 16 and the signal line 12 when a voltage is applied to the fixed electrode 14 is shown in FIG. FIG. 8 is a numerical value obtained by the equation C = Sεk / d. C indicates the capacitance between the movable electrode 16 and the signal line 12. S indicates the facing area of the movable electrode 16 and the signal line 12. ε is a dielectric constant of vacuum, and here, 8.85E-12F / m is used. k is a relative dielectric constant, which is 1 here. The dielectric constant of the SiN layer on the signal line 12 is set to 1 for convenience of calculation.

  As shown in FIG. 6, when a voltage of 0 V is applied to the fixed electrode 14, the displacement of the movable electrode 16 in the z direction is 0 μm, and the capacitance of the signal line 12 and the movable electrode 16 is 0.177 fF. When a voltage of 65 V is applied to the fixed electrode 14, the displacement of the movable electrode 16 in the z direction is about 0.4 μm, and the capacitance between the signal line 12 and the movable electrode 16 becomes the maximum value, which is about 8 fF. Therefore, when the applied voltage is 0 V and 65 V, the device 10 can take a binary capacity of 0.177 fF and 8 fF. Moreover, the capacity | capacitance of the device 10 can also be changed continuously by changing an applied voltage continuously between 0V-65V.

  As shown in FIG. 6A, a voltage V is first applied from a power source 40 to the fixed electrode 14. The movable electrode 16 is grounded.

  At this time, as shown in FIG. 6C, the movable electrode 16 receives an electrostatic force as indicated by an arrow 15 from the fixed electrode 14, but the fixed electrode 14 is in a symmetrical position with the movable electrode 16 in between, so that the movable electrode 16 is movable. The forces in the y direction that the electrodes 16 receive from the fixed electrodes 14 on both sides are balanced because they have the same magnitude and the opposite directions. Also, as shown in FIG. 6B, since the x direction is also symmetric, the force in the x direction that the movable electrode 16 receives from the fixed electrode 14 is also balanced. As a result, the movable electrode 16 does not move in the xy direction, and the distance from the fixed electrode 14 does not vary.

  As shown in FIG. 6C, the movable electrode 16 is attracted to the signal line 12 by receiving an electrostatic force from the fixed electrode 14 as indicated by an arrow F on the surface of the movable electrode 16 facing the signal line 12. Electrostatic force works in the direction. As a result, as shown in FIG. 7, as the applied voltage V increases, the movable electrode 16 is displaced in the Z direction so as to be negative. That is, the signal line 12 is drawn.

  Here, as shown in FIG. 7, as the applied voltage V increases, the capacitance between the movable electrode 16 and the signal line 12 increases. When the movable electrode 16 is closest to and contacts the signal line 12, the capacity between them is the largest. However, when the applied voltage applied to the fixed electrode 14 is V1, the applied voltage V1 and no applied voltage are present. (0V) can be a binary capacitance change.

  Further, as shown in FIG. 7, the capacitance is continuously changed by continuously changing the applied voltage V between 0 and V1, so that it is possible to adjust the applied voltage V appropriately and continuously. It can also be a change in value capacity.

  In the structure according to this embodiment, even if the movable electrode 16 moves, the distance between the fixed electrode 14 and the movable electrode 16 does not vary. For this reason, the fixed electrode 14 and the movable electrode 16 are not in contact with each other, so that sticking, which has been a problem in the conventional structure, does not occur. Therefore, no sticking measures such as IBA driving are required, and a reduction in the number of components, cost reduction, and power saving can be expected. In addition, the substrate, the electric circuit, etc. including the device 10 can be reduced in size.

<Second Embodiment>
An example of the MEMS tunable capacitor according to the second embodiment of the present invention is shown in FIGS. It should be noted that the numerical values such as the dimensions shown are examples, and the present invention is not limited to these in practice, and can be implemented in various modes without departing from the scope of the invention.

  As shown in FIGS. 9 and 10, the device 11 that is the MEMS tunable capacitor according to the second embodiment of the present invention includes a signal line 12 formed on the surface (mounting surface) P of the substrate, and a signal line 12. Two pairs or more of the columnar anchors 20A standing about the center of the signal line, and two pairs or more of the anchors 20A standing so as to sandwich the signal line 12 from the outside of both ends in the longitudinal direction of the signal line 12 A plurality of anchors 20B having the same shape and a plurality of standing in a comb-like structure as a whole at a position sandwiching the signal line 12 along the longitudinal direction of the signal line 12 so as to be orthogonal to the center line in the width direction of the signal line 12 A signal line is formed in the space above the signal line 12 by the plate-shaped fixed electrode 14 formed, the spring 18A extending from the anchor 20A, and the spring 18B extending from the anchor 20B. And a plate having a comb-like structure as a whole, which is provided so as to be alternately inserted between the variable capacitance electrode 50 and the individual fixed electrodes 14. And a movable electrode 16 having a shape.

  The anchor 20 </ b> A is fixed at an outer position of the fixed electrode 14 in the vicinity of both ends in the longitudinal direction of the signal line 12, and a spring 18 </ b> A is provided at the tip portion toward the signal line 12. The anchor 20B is fixed further outward in the longitudinal direction than both ends of the signal line 12 in the longitudinal direction, and a spring 18B is provided at the distal end in the direction along the signal line 12.

  A pair of springs 18 </ b> A extending from the pair of anchors 20 </ b> A toward the signal line 12 is connected to a variable capacitance electrode 50 provided along the longitudinal direction of the signal line 12 in the space above the signal line 12. Similarly, the spring 18B provided at the tip of the anchor 20B is connected to the longitudinal end portion of the variable capacitance electrode 50 to suspend it.

  The movable electrodes 16 are arranged in the longitudinal direction so as to protrude in the width direction of the variable capacitance electrode 50, and the size (thickness) in the contact / separation direction from the signal line 12 is equal to the thickness of the variable capacitance electrode 50. The movable electrodes 16 having a comb-like shape as a whole are arranged in a staggered manner so as to be inserted between the fixed electrodes 14 having the same comb-teeth shape. The parts overlap.

  As shown in FIG. 10, the tip of the movable electrode 16 in the z direction (end farther from the surface P) than the height of the fixed electrode 14 provided on the surface P in the protruding direction (z direction) from the surface P is z. It protrudes higher in the direction, and the movable electrode 16 is held at a position far from the surface P between the fixed electrodes 14 in a side view as shown in FIGS.

  The variable capacitance electrode 50 and the movable electrode 16 have substantially the same size when viewed from the longitudinal direction (x direction) of the signal line 12, that is, the size (thickness) in the z direction and the signal line 12. The widths in the orthogonal direction (y direction) are substantially equal.

  The springs 18A and 18B for suspending the variable capacitance electrode 50 have a large size in the direction (xy direction) along the surface P, and the thickness in the z direction is thinner than the xy direction. Therefore, the springs 18A and 18B are suspended by the springs 18A and 18B. The variable capacitance electrode 50 is not easily moved in the xy direction, and is held easily in the z direction that is in contact with and away from the surface P.

  That is, the variable capacitance electrode 50 and the movable electrode 16 provided so as to protrude from the variable capacitance electrode 50 in the width direction are not contacted with or separated from the fixed electrode 14 in the direction along the surface P (xy direction), and the signal line. 12 is movable in the contact / separation direction (the direction of arrow L in FIG. 2).

  An insulating layer 30 and an insulating layer 32 are respectively provided on the outer peripheral surface of the signal line 12 and the surface of the fixed electrode 14 on the side facing the movable electrode 16, and the signal line 12, the movable electrode 16, and the fixed electrode 14, respectively. Even if it contacts with the movable electrode 16, it is set as the structure which is not short-circuited.

<Example of Manufacturing Process of Second Embodiment>
11 and 12 show an example of a process for forming a device which is an example of a MEMS tunable capacitor according to the second embodiment of the present invention. The materials and numerical values shown are examples, and the present invention is not limited to these at the time of implementation. The present invention can be implemented in various modes without departing from the gist.

  First, as shown in FIG. 11A, a 100 nm WSi layer 102 for forming a signal line 12 is formed on a silicon substrate 100 by a known method (for example, film formation by sputtering). 11 and 12 show the B-B ′ cross section shown in FIG. 10C and the A-A ′ cross section shown in FIG. 10B side by side.

  Next, as shown in FIG. 11B, the WSi layer 102 is etched, leaving the WSi layer 102A that becomes the signal line 12.

  Further, as shown in FIG. 11C, POLy-Si 104 is formed to a thickness of 1 μm by a known method (for example, film formation by LP-CVD).

  Next, as shown in FIG. 11D, POLy-Si 104 is etched, and the remaining portion is formed as a fixed electrode 14.

  Further, as shown in FIG. 11E, a SiN insulating layer 106 is deposited to a thickness of 0.1 μm by a known method (for example, film formation by LP-CVD).

  Next, as shown in FIG. 11F, the SiN insulating layer 106 is etched, the insulating layer 30 is formed on the outer peripheral surface of the signal line 12, and the insulating layer 32 is formed on the surface of the fixed electrode 14 facing the movable electrode 16. . Since there is a surface that does not face the movable electrode 16 only at the extreme end of the fixed electrode 14 (for example, the upper right end in FIG. 10A), the surface that faces the movable electrode 16 without necessarily providing the insulating layer 32 on both surfaces. Only the insulating layer 32 may be provided.

Further, as shown in FIG. 11G, a SiO ₂ layer 108 as a sacrificial layer is formed to a thickness of 1.7 μm by a known method (for example, film formation by PE-CVD).

Next, as shown in FIG. 12A, the primary etching of the SiO ₂ layer 108 is performed with a thickness of 1 μm. Thereby, the space 120 used as the anchor 20 is created.

Further, as shown in FIG. 12B, the secondary etching of the SiO ₂ layer 108 is performed with a thickness of 1.2 μm. As a result, a space 116 that becomes the movable electrode 16 and a space 150 that becomes the variable capacitance electrode 50 are created.

Next, as shown in FIG. 12C, the third etching of the SiO ₂ layer 108 is performed with a thickness of 0.2 μm. As a result, a space 180 serving as the spring 18A and the spring 18B is created.

  Further, as shown in FIG. 12D, POLy-Si 110 for forming the variable capacitance electrode 50, the movable electrode 16, the anchor 20A, and the anchor 20B is formed to a thickness of 1 μm by a known method (for example, film formation by LP / CVD).

  Next, as shown in FIG. 12E, POLy-Si 110 is polished and planarized by a CMP (Chemical Mechanical Polishing) process or the like.

  Finally, as shown in FIG. 12F, POLy-Si 110 is removed. For example, POLy-Si110 is removed by immersing in 5% HF. By these steps, the variable capacitance electrode 50 and the movable electrode 16 suspended by the spring 18A and the spring 18B are positioned on the signal line 12 covered with the insulating layer 30 as shown in FIG. A structure in which the fixed electrode 14 is erected on both sides of the line 12 can be adopted.

<Operation / Effect of Second Embodiment>
The operation of the MEMS tunable capacitor according to the second embodiment of the present invention will be described with reference to FIG.

  In order for the variable capacitance electrode 50 to be movable with respect to the signal line 12 without changing the distance between the movable electrode 16 of the device 11 shown in FIG. 10 and the fixed electrode 14, the movable electrode 16 and the variable capacitance electrode 50 are shown in FIG. It needs to have a shape that is easy to move in the middle z direction, that is, a direction that is close to and away from the surface P of the substrate, and that is difficult to move in the x direction and the y direction that are directions along the surface P of the substrate.

  In the following, k is the spring coefficient of the spring 18A and spring 18B (cantilever), E is the Young's modulus of the material of the spring 18A and spring 18B (about 160 GPa for POLySi), b is the width of the spring 18A and spring 18B, and t is The thicknesses of the springs 18A and 18B, 1 is the length of the springs 18A and 18B. kp indicates the spring coefficient of the springs 18A and 18B connected to the movable electrode 16 in parallel. kt indicates the overall spring coefficient when the spring 18A and the spring 18B are connected in series.

  When the length of the variable capacitance electrode 50 is 200 μm as in the example shown in FIG. 10, a total of 200 structures such as the A-A ′ cross section shown in FIG. 10B can be formed. When the pitch of the springs 18A is 20 μm, a total of 22 springs 18A are necessary, and the spring coefficient that the springs 18A act on the variable capacitance electrode 50 is considered as an integral body of the spring 18A and the movable electrode 16 connected to the tip thereof. There is a need. In addition, the four springs 18B also affect the operation of the variable capacitance electrode 50.

  As shown in FIG. 10, since the spring 18A and the movable electrode 16 are connected in series, the overall spring coefficient kt can be obtained by Equation 1 as in the first embodiment. In the example of FIG. 10, the length of the spring 18A is 40 μm, the width is 0.5 μm, and the thickness is 0.2 μm. According to the equation (1), the spring coefficient kz in the z direction is 0.0025 N / m, The spring coefficient ky is about 4 GN / m.

  The spring coefficient of the movable electrode 16 connected in series with the spring 18A is 25 μm in length, 0.5 μm in width, and 1 μm in thickness, kz is 1.28 N / m, and ky is 125 MN / m. Therefore, ktz is about 0.0025 N / m, and kty is about 121 MN / m.

  There are 22 portions where the spring 18A and the movable electrode 16 are connected in series, and all of them are connected to the variable capacitance electrode 50. Therefore, when calculated by the equation (2) of page 8, it acts on the variable capacitance electrode (8). kpz is 0.055 N / m and kpy is about 2.5 GN / m. Thus, since the spring coefficient in the y direction is high, the variable capacitance electrode 50 and the movable electrode 16 are difficult to move in the y direction.

  As shown in FIG. 10, the dimensions of the spring 18B are 30 μm in length, 1 μm in width, and 0.2 μm in thickness. In this case, the z-direction spring coefficient is kz = 0.012 N / m when calculated by Equation (1) of [Equation 1]. When kx≈400 MN / m, ky = 0.3 N / m, and since the four springs 18B are connected in parallel to the variable capacitance electrode 50, kz = 0.048 N / m, kx = l. 6 GN / m. Thus, since the spring coefficient in the x direction is high, the variable capacitance electrode 50 and the movable electrode 16 are difficult to move in the x direction.

  From the above description, the z-direction spring coefficient kz acting on the variable capacitance electrode 50 is a spring coefficient obtained by combining the four springs 18B in addition to the series springs of the 22 springs 18A and the movable electrode 16. That is, kz = 0.055 N / m + 0.048 N / m = 0.103 N / m.

  As described above, in order to make the variable capacitance electrode 50 move easily in the z direction, the shape of the spring 18A and the spring 18B is such that the width in the direction along the surface P of the substrate is close to and away from the surface P of the substrate. It is necessary that the thickness is sufficiently larger than the thickness in the direction in which the film is formed.

  In the above example, the ratio of the width of the spring 18B in the direction along the surface P of the substrate to the thickness in the direction of contact with and away from the surface P of the substrate is 5, and the spring coefficient in the direction along the surface P of the substrate is z direction. The spring coefficient is about 29 times or more.

  When the thickness ratio is 2, the x-direction spring coefficient kx is four times the z-direction spring coefficient kz. The inventors have obtained from the experiment knowledge that it is determined that the selection ratio in the x direction and the z direction can be obtained even at this magnification. For this reason, in order to make the movable electrode 16 and the variable capacitance electrode 50 easily move in the z direction and difficult to move in the x direction and the y direction, the direction along the surface P of the substrate of the spring 18A and the spring 18B (x direction, It is desirable that the ratio of the width in the y direction) and the size (thickness) in the z direction perpendicular thereto is at least 2 or more.

  Next, the operation of the MEMS tunable capacitor according to the second embodiment of the present invention is shown in FIGS.

  FIG. 13 shows the relationship between the displacement of the movable electrode 16 with respect to the signal line 12 and the applied voltage, and FIG. 14 shows the relationship between the capacitance change between the movable electrode 16 and the signal line 12 and the voltage. That is, in the device 11 shown in FIG. 10, when a voltage of 0V to 3V is applied to the fixed electrode 14, the amount of displacement in the z direction of the variable capacitance electrode 50 is shown in FIG. FIG. 14 shows a change in capacitance between the variable capacitance electrode 50 and the signal line 12 when a voltage is applied to the fixed electrode 14.

  In operation, a voltage V is applied to the device 11 to the fixed electrode 14 shown in FIG. The movable electrode 16 and the variable capacitance electrode 50 are grounded. In this case, the electric lines of force received by the movable electrode 16 from the fixed electrode 14 are shown in the electric lines of force in FIG. 6C, as in the first embodiment.

  Since the movable electrode 16 has the same spacing as the fixed electrodes 14 on both sides, the horizontal electrostatic force received from the fixed electrodes 14 on both sides is balanced because the magnitude is the same and the direction is opposite. For this reason, the movable electrode 16 does not move in the direction (x direction) along the surface P of the substrate, and the distance from the fixed electrode 14 does not change.

  In the direction in which the movable electrode 16 is in contact with or away from the surface P of the substrate (z direction), the surface of the movable electrode 16 facing the substrate receives electrostatic force from the fixed electrode 14, and the direction of electrostatic force is directed from the movable electrode 16 to the substrate. Due to this electrostatic force, the movable electrode 16 moves toward the surface P of the substrate. Since the movable electrode 16 is integrally fixed to the variable capacitance electrode 50, the variable capacitance electrode 50 moves toward the signal line 12. The interval between the variable capacitance electrode 50 and the signal line 12 is reduced, and the capacitance with the signal line 12 is increased.

  When the variable capacitance electrode 50 comes into contact with the signal line 12, the capacitance becomes the maximum value. When the applied voltage to the fixed electrode 14 is V1, the applied voltage is binary between V1 and 0V (no voltage applied). Capacity can be taken. Further, since the capacitance continuously changes when the voltage applied to the fixed electrode 14 is 0 to V1, a multi-value capacitance can be obtained.

  Specifically, as shown in FIG. 13, when a voltage of 0 V is applied to the fixed electrode 14, the displacement in the z direction of the movable electrode 16 is 0 μm (does not move), and the signal line 12 and the variable capacitance electrode 50 The capacitance is 0.5 pF.

  When a voltage of 3 V is applied to the fixed electrode 14, the displacement amount of the movable electrode 16 in the z direction is about 0.5 μm, and the capacitance between the signal line 12 and the variable capacitance electrode 50 at this time is about 3.8 pF at the maximum. Therefore, the device 11 can take a binary capacitance of 0.5 pF and 3.8 pF with the applied electrodes of 0 V and 3 V.

  Further, as shown above, as shown in FIG. 14, by continuously changing the applied voltage V between 0V and 3V, the capacity also continuously changes. Therefore, the applied voltage V is appropriately adjusted and continuously changed. By doing so, it is possible to make a multi-value capacity change.

  Furthermore, in this embodiment, the capacitance of the device 11 does not depend on the size of the movable electrode 16 and is determined by the area of the variable capacitance electrode 50. Therefore, by adjusting the area of the variable capacitance electrode 50, the maximum capacitance of the device 11 is increased. It becomes possible to enlarge compared with 1 embodiment. In addition, the rate of change in capacitance (maximum capacitance / minimum capacitance) can also be made larger than in the first embodiment by adjusting the area of the variable capacitance electrode 50. Further, the operating voltage can be made lower than that in the first embodiment.

  In the structure according to the present embodiment, the distance between the fixed electrode 14 and the movable electrode 16 does not change even if the variable capacitance electrode 50 moves. For this reason, the fixed electrode 14 and the movable electrode 16 are not in contact with each other, so that sticking, which has been a problem in the conventional structure, does not occur. Therefore, no sticking measures such as IBA driving are required, and the reduction in the number of components, cost reduction, and power saving can be expected. In addition, the substrate including the device 10 and the electric circuit can be downsized. This is the same as in the first embodiment. Furthermore, since the operating voltage can be made lower than in the first embodiment, it is easy to operate with a battery of a mobile phone.

<Third Embodiment>
An example of a MEMS tunable capacitor according to the third embodiment of the present invention is shown in FIG. It should be noted that the numerical values such as the dimensions shown are examples, and the present invention is not limited to these in practice, and can be implemented in various modes without departing from the scope of the invention.

  As shown in FIG. 15, the device 11 </ b> B that is the MEMS tunable capacitor according to the third embodiment of the present invention includes a signal line 12 formed on the surface (mounting surface) P of the substrate, and the signal line 12 as a center. Two pairs or more of the columnar anchors 20A and substantially the same shape as the two pairs or more of the anchors 20A erected so as to sandwich the signal lines 12 from outside the both ends in the longitudinal direction of the signal lines 12. A plurality of plates that stand in a comb-like structure as a whole so as to be perpendicular to the center line in the width direction of the signal line 12 at a position sandwiching the signal line 12 along the longitudinal direction of the anchor 20C and the signal line 12 The point which is equipped with the fixed electrode 14 of the shape is the same as that of 2nd Embodiment.

  In the present embodiment, a spring 18A extending from the anchor 20A and a spring 18D extending from the center in the longitudinal direction of the spring 18C connecting the tips of the anchor 20C so as to straddle the signal line 12 extend along the signal line 12. And a variable capacitance electrode 50 suspended so as to cover the signal line 12 in the space above the signal line 12.

  Similarly to the second embodiment, plate-like movable electrodes 16 that are provided so as to be alternately inserted between the variable capacitance electrodes 50 and the individual fixed electrodes 14 and have a comb-like structure as a whole are provided. I have.

  The anchor 20 </ b> A is fixed at an outer position of the fixed electrode 14 in the vicinity of both ends in the longitudinal direction of the signal line 12, and a spring 18 </ b> A is provided at the tip portion toward the signal line 12. The anchor 20C is fixed further outside in the longitudinal direction and outside in the width direction than both longitudinal ends of the signal line 12, and a spring 18C is provided at the distal end so as to connect the pair of anchors 20C provided at both longitudinal ends of the signal line 12. Is provided.

  A pair of springs 18 </ b> A extending from the pair of anchors 20 </ b> A toward the signal line 12 is connected to a variable capacitance electrode 50 provided along the longitudinal direction of the signal line 12 in the space above the signal line 12. The spring 18C connecting the tip of the anchor 20C is connected to the longitudinal end of the variable capacitance electrode 50, and the spring 18D connecting the variable capacitance electrode 50 is connected to the longitudinal end of the variable capacitance electrode 50. Suspended. That is, the spring 18D extending from the center in the longitudinal direction of the spring 18C connecting the anchors 20C toward the center in the longitudinal direction of the variable capacitance electrode 50 is substantially T-shaped together with the spring 18C. 50 longitudinal ends are supported. Since the structure of other parts is the same as that of the second embodiment, description thereof is omitted.

<Operation and Effect of Third Embodiment>
As shown in FIG. 15, the device 11 </ b> B according to the third embodiment of the present invention is supported in the longitudinal direction by a single spring 18 </ b> D extending from the center of both ends in the longitudinal direction of the variable capacitance electrode 50. .

  While the variable capacitance electrode 50 of the device 11 according to the second embodiment is supported in the longitudinal direction by the two springs 18B, in the present embodiment, the variable capacitance electrode 50 is supported by one spring 18D and the end of the spring 18D is Since the spring 18C is connected to the center of the spring 18C, for example, a spring having a large (thick) thickness in the direction (z direction) contacting and separating from the surface P of the substrate is obtained in order to obtain the same spring coefficient as the spring 18B of the second embodiment. Can be used.

  That is, the spring coefficient equivalent to that of the spring 18B can be obtained while using the thicker spring 18C and spring 18D, so that it is not necessary to increase the processing accuracy compared to the spring 18B, and in addition, the mechanical strength is higher than that of the spring 18B. High spring 18C and spring 18D can be used.

<Summary>

  As mentioned above, although the Example of this invention was described, it cannot be overemphasized that this invention is not limited to said Example at all, and can implement in a various aspect in the range which does not deviate from the summary of this invention.

  That is, in the above embodiment, the plate-like movable electrode or variable capacitance electrode is suspended on a single signal line. However, the present invention is not limited to this. For example, a plurality of movable electrodes or variable capacitance electrodes are combined. Or may be combined with other devices.

It is a conceptual diagram which shows the example of the apparatus using the MEMS tunable capacitor which concerns on this invention. 1 is a perspective view showing a MEMS tunable capacitor according to a first embodiment of the present invention. It is a three-plane figure which shows the structure of the MEMS tunable capacitor which concerns on the 1st form of this invention. It is sectional drawing which shows the formation process of the MEMS tunable capacitor which concerns on the 1st form of this invention. It is sectional drawing which shows the formation process of the MEMS tunable capacitor which concerns on the 1st form of this invention. FIG. 3 is a three-sided view showing the operation of the MEMS tunable capacitor according to the first embodiment of the present invention. It is a graph which shows the operation | movement of the MEMS tunable capacitor which concerns on the 1st form of this invention. It is a graph which shows the operation | movement of the MEMS tunable capacitor which concerns on the 1st form of this invention. It is a perspective view which shows the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is a three-view figure which shows the structure of the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is sectional drawing which shows the formation process of the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is sectional drawing which shows the formation process of the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is a graph which shows the operation | movement of the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is a graph which shows the operation | movement of the MEMS tunable capacitor which concerns on the 2nd form of this invention. It is a three-view figure which shows operation | movement of the MEMS tunable capacitor which concerns on the 3rd form of this invention.

10 Device 11 Device 12 Signal Line 14 Fixed Electrode 16 Movable Electrode 18 Spring 20 Anchor 30 Insulating Layer 32 Insulating Layer 50 Variable Capacitance Electrode

Claims (12)

A signal line provided on the substrate;
A pair of fixed electrodes provided on the substrate and electrically connected across the signal line, and having a rectangular parallelepiped shape,
A movable electrode suspended between the fixed electrode facing the signal line and capable of contacting and separating from the signal line,
A MEMS tunable capacitor, wherein a capacitance is changed by changing a distance between the variable electrode and the signal line by an electrostatic force generated by providing a potential difference between the fixed electrode and the movable electrode. 2. The MEMS tunable capacitor according to claim 1, wherein an end portion of the movable electrode far from the substrate protrudes from an end portion of the fixed electrode far from the substrate. 3. The MEMS according to claim 1, wherein the movable electrode is suspended by an elastic member having higher rigidity in the direction along the surface of the substrate than in the direction along the surface of the fixed electrode. Tunable capacitor. 4. The MEMS tunable capacitor according to claim 3, wherein the elastic member has a width in a direction along the substrate larger than a thickness in a direction orthogonal thereto. 5. The MEMS tunable capacitor according to claim 1, wherein a surface of the signal line and surfaces of the fixed electrode and the movable electrode facing each other are covered with an insulating film. . A signal line provided on the substrate;
A plurality of fixed electrodes provided in a row on the substrate across the signal line and electrically connected;
A space on the substrate facing the signal line, a variable capacitance electrode suspended so as to be in contact with and away from the signal line,
A movable electrode provided integrally with the variable capacitance electrode and positioned between the fixed electrodes,
A MEMS tunable capacitor, wherein the capacitance is changed by changing a distance between the variable capacitance electrode and the signal line by an electrostatic force generated by setting a potential difference between the fixed electrode and the variable capacitance electrode. The variable capacitance electrode has a rectangular parallelepiped shape provided along the substrate,
The fixed electrode is a comb-like structure in which a plurality of thin plate-like members having the same shape standing on both sides of the signal line are arranged at regular intervals so as to face each other,
The movable electrodes are arranged such that a plurality of thin plate-like members provided on the variable capacitance electrode at regular intervals with the fixed electrode alternately enter between the fixed electrodes one by one, and face each other with a gap therebetween. Comb structure,
The size of the movable electrode in the direction perpendicular to the substrate is equal to the thickness of the variable capacitance electrode in the contact / separation direction from the substrate,
The end of the movable electrode and the variable capacitance electrode on the side far from the substrate is farther from the substrate than the end of the fixed electrode on the side far from the substrate. MEMS tunable capacitor. 8. The variable capacitance electrode according to claim 6, wherein the variable capacitance electrode is suspended by an elastic member having a rigidity in a direction along the surface of the substrate higher than a rigidity in a direction along the surface of the fixed electrode. MEMS tunable capacitor. 9. The MEMS tunable capacitor according to claim 8, wherein the elastic member has a width in a direction along the substrate larger than a thickness in a direction orthogonal thereto. The variable capacitance electrode is supported by a plurality of the elastic members provided in a direction along the signal line and a plurality of the elastic members provided in a direction intersecting with the elastic members. 8. The MEMS tunable capacitor according to 8. The surface of the signal line, the surface of the variable capacitance electrode, and the surfaces of the fixed electrode and the movable electrode facing each other are covered with an insulating film. The MEMS tunable capacitor according to claim 1. The MEMS tunable capacitor according to claim 1, wherein a voltage is applied to the fixed electrode and the movable electrode is grounded.

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